1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV), and in particular to controlling torque transmitted by the output of the powertrain to the vehicle wheels while executing a transmission shift.
2. Description of the Prior Art
In a conventional vehicle with a fixed-ratio transmission, the driver can experience driveline disturbances during a transmission shift event, i.e., an upshift or a downshift. The driveline disturbances occur due to the acceleration and deceleration of engine and transmission components, which acceleration and deceleration produce an inertial torque during the shift event. In the case of an upshift, the transmission output torque increases during the ratio change phase, i.e., inertia phase, of the shift as a result of the engine speed changing, as shown in FIG. 1 at point 12. This output torque disturbance is felt by the vehicle's occupants and severely degrades shift quality.
The magnitude of the output shaft torque disturbance increases the faster the upshift is performed, since the magnitude of engine deceleration is greater. By reducing engine torque produced during the upshift, as shown at point 14, the inertial torque can be offset and the output shaft torque increase can be minimized, as shown at point 16, thereby improving the quality of the shift. This method described with reference to FIG. 1 is referred to as “input torque modulation” control.
In the case of a downshift, the transmission output torque decreases during the ratio change phase as the engine and transmission components accelerate to the synchronous speed for the lower gear, as shown in FIG. 2 at point 18. Moreover, as shown at point 20 during the torque transfer phase, the transmission output torque can spike near the completion of the downshift as the engine accelerates. The drop in output torque during the ratio change phase is felt by vehicle's occupants and can give the sense of an acceleration discontinuity as the downshift is performed. The output torque spike at the end of the downshift can degrade shift quality and give the occupants a feeling of a harsh or rough shift. Furthermore, the magnitude of output shaft torque drop and spike near the end of the downshift increases in proportion to speed of the downshift. By using input torque modulation, the engine combustion torque is reduced near the end of the downshift, as shown at point 22, in order to reduce the engine's acceleration as the shift ends. As a result, the transmission output torque spike can be minimized and avoided, as shown at point 24, thereby reducing the shift disturbance.
In conventional vehicle applications, the problems that can occur with input torque modulation during shifts include limited engine torque reduction authority due to other constraints such as emissions, delayed or poor engine torque response to torque modulation requests, further degrading shift quality; and wasted fuel energy and efficiency since spark retardation is commonly used for achieving torque modulation requests.